In the related art, a sensor using a silicon on insulator (SOI) substrate in which a support substrate, an insulating film, and a semiconductor layer are laminated in this order has been proposed as a capacitance type physical quantity sensor (for example, see Patent literature 1).
Specifically, in this capacitance type physical quantity sensor, a movable unit including multiple movable electrodes which can cause displacement in a predetermined direction is formed on the semiconductor layer. In addition, a first fixed unit which includes a first support unit including a first fixed electrode opposing the movable electrode, and a second fixed unit which includes a second support unit including a second fixed electrode opposing the movable electrode and is disposed on a side opposite to the first fixed unit with the movable unit interposed therebetween, are formed on the semiconductor layer. That is, a pair of the first and second fixed units are formed on the semiconductor layer with the movable unit interposed therebetween.
A recessed portion is formed on a part including parts of the supporting substrate and the insulating film opposing the movable electrodes and the first and second fixed electrodes, and the movable electrodes and the first and second fixed electrodes are in a floating state. In order to cause the first and second fixed electrodes to completely float, end portions of the first and second support units including the first and second fixed electrodes on the movable unit side are partially protruded to the recessed portion.
In such a capacitance type physical quantity sensor, first capacitance is configured of detection capacitance between the movable electrode and the first fixed electrode and parasitic capacitance between the first fixed unit and the supporting substrate. In the same manner as described above, second capacitance is configured of detection capacitance between the movable electrode and the second fixed electrode and parasitic capacitance between the second fixed unit and the supporting substrate. Physical quantity is detected based on a difference between the first capacitance and the second capacitance.
The magnitude of the parasitic capacitance is proportional to areas of portions of the first and second fixed units that are bonded with the supporting substrate through the insulating film. Accordingly, the areas of portions of the first and second fixed units that are bonded with the supporting substrate are set to be equivalent to each other, so that each parasitic capacitance is canceled out when a difference between the first and second capacitance is calculated.
The capacitance type physical quantity sensor is, for example, produced as follows. That is, the insulating film is formed on the supporting substrate and the recessed portion is formed on the supporting substrate and the insulating film. After that, the semiconductor layer configured with a silicon substrate is bonded to the insulating film. A mask is formed on the semiconductor layer and the mask is patterned. Then, reactive ion etching (i.e., RIE) or the like is performed to form the movable electrodes and the first and second fixed electrodes, and the capacitance type physical quantity sensor is produced.
However, in the capacitance type physical quantity sensor, when forming the movable unit and the first and second fixed units, position shift or the like may occur when patterning the mask, and the movable units and the first and second fixed units may be generally shifted from formation expectation areas. For example, when the first fixed unit, the movable unit, and the second fixed unit are generally shifted to the first fixed unit side in an arrangement direction of the first fixed unit, the movable unit, and the second fixed unit, the area of the end portion of the first support unit on the movable unit side which is protruded on the recessed portion is decreased. With respect to this, the area of the end portion of the second support unit on the movable unit side which is protruded on the recessed portion is increased.
That is, the area of the first fixed unit (i.e., first support unit) which is bonded with the supporting substrate through the insulating film is increased and the area of the second fixed unit (i.e., second support unit) which is bonded with the supporting substrate through the insulating film is decreased. That is, the parasitic capacitance formed between the first fixed unit and the supporting substrate is increased and the parasitic capacitance formed between the second fixed unit and the supporting substrate is decreased. Accordingly, when calculating a capacitance difference between the first capacitance and the second capacitance, each parasitic capacitance is not canceled out and a detection error may be generated.
This problem occurs in the same manner even in a capacitance type physical quantity sensor including a cap portion for covering the movable electrodes and the first and second fixed electrodes, in order to prevent attachment of foreign materials to the movable electrodes and the first and second fixed electrodes. That is, in such a capacitance type physical quantity sensor, the cap portion is configured by forming the insulating film on the semiconductor substrate and the semiconductor substrate is bonded to the semiconductor layer through the insulating film. A recessed portion is formed on a part including parts of the semiconductor substrate and the insulating film opposing the movable electrodes and the first and second fixed electrodes. In addition, parasitic capacitance is formed between the first and second support units and the semiconductor substrate, respectively. Accordingly, if position shift occurs due to alignment shift when bonding the semiconductor substrate (i.e., cap portion) and the semiconductor layer, the areas of portions of the first and second fixed units that are bonded with the semiconductor substrate (i.e., cap portion) are different from each other and parasitic capacitance thereof are different from each other.
A SOI substrate in which a thermal oxide film is formed on a first substrate including one surface where a recessed portion is formed, and a second substrate is disposed on one surface side of the first substrate with the thermal oxide film interposed therebetween has been proposed (for example, see Patent literature 2). In the SOI substrate, a silicon substrate is used as the first substrate.
The SOI substrate is produced as follows. First, the recessed portion is formed on one surface of the first substrate. Then, the first substrate is subjected to thermal oxidation to form a thermal oxide film. At that time, the thermal oxide film is formed on the entire surface of the first substrate and also formed on a wall surface of the recessed portion. The SOI substrate is produced by bonding the thermal oxide film formed on one surface of the first substrate and the second substrate.
In the SOI substrate, when forming a sensing unit which outputs a sensor signal according to physical quantity by performing dry etching on the second substrate, the thermal oxide film formed on the recessed portion can be used as an etching stopper. Accordingly, a rough bottom surface of the recessed portion can be prevented. In addition, by forming the thermal oxide film (insulating film) on the recessed portion, the electrical connection between the first substrate and the second substrate through foreign materials can be prevented.
However, in the SOI substrate, when the first substrate is subjected to thermal oxidation to form the thermal oxide film, stress is concentrated on an opening of the recessed portion. Accordingly, the portion of the thermal oxide film formed on the opening of the recessed portion has a greater film thickness than that of the portion thereof formed on other areas. That is, on one surface of the first substrate, the portion of the thermal oxide film formed on the opening of the recessed portion is formed to be swollen, compared to the portion thereof formed on other areas. Accordingly, when the thermal oxide film and the second substrate are bonded to each other in such a state, only the swollen portion of the thermal oxide film is bonded with the second substrate, and thus, adhesiveness of the thermal oxide film and the second substrate is deteriorated.